The invention relates broadly to an apparatus and method for measuring the expansion properties of cement compositions which are capable expanding during a curing or setting period. In a specific application, the apparatus is used to measure the expansion properties of a cement slurry composition, such as the type of material used in cementing of oil or gas wells.
In a typical well cementing operation, a cement slurry is run into the annulus between the well casing and the bore hole, frequently at more than one location in the bore hole. As the cement slurry hardens to a solid during the setting-up period, it is essential that the cement composition expand sufficiently to provide a good bond with the well casing and also the wall of the bore hole. Otherwise, if the cement should shrink during hardening, it can leave channels between the bore hole wall and the cement column and between the cement column and the well casing. This "channeling" effect is undesirable for several reasons. One reason is that gas or oil from a producing formation could leak into these channels and thus by-pass the production tubing which carries it to the well head.
Because of the problems mentioned above, it would be desirable to have a reliable procedure for measuring the expansion properties of cement compositions. The prior tests for determining expansion of cement compositions have not been entirely satisfactory. For example, one of the procedures in common use is a test derived by the American Society for the Testing of Materials, in which the cement slurry is put into a bar-type mold and cured underwater and at atmospheric pressure. During the curing cycle, the first time the cement bar is removed from the mold is when it is hard enough so that it won't break up as it comes out of the mold. After the bar is removed from the mold, it is carefully measured for length and then returned to the water curing bath to allow it to finish curing. Periodically, during the curing period, the bar is removed from the water bath and the length is measured each time.
The ASTM test has a major drawback. For example, when the bar is first removed from the mold, it is assumed that the cement has not cured long enough for the material to begin to expand. However, in reality, this point in time does not exist. Present studies clearly indicate that a cement composition will begin to expand from the moment it starts to set-up. In the prior procedure, therefore, it is virtually impossible to obtain reliable data on the expansion properties of cement compositions.